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Airframes, Engines & Systems 57
After flight, many engines have a
rundown period which must be strictly
observed if you want to keep it for
any length of time. As engines get
smaller relative to power output,
they have to work harder. Also, in
turbines, there are no heavy areas to
act as heat sinks, like the fins on a
piston engine, which results in
localised hotspots which may
deform, but are safe if cooled
properly, with the help of circulating
oil inside the engine (75% of the air
taken into a turbine is for cooling
purposes). If you shut down too
quickly, the oil no longer circulates,
which means that it may carbonise
on the still-hot surfaces, and build
up enough to prevent the relevant
parts from turning. This coking up
could sieze the engine within 50
hours or less.
If the starter light remains on after
you release the starter button on a
piston engine, you should shut it
down, as it indicates that the starter
is still engaged with the engine and is
being driven by it.
Lubrication
Friction can be quite handy, but not
inside an engine. Without some way
of making the various surfaces rub
smoothly against each other, they
would get hot, and suffer from
scoring damage.
Oil actually does many things,
including cooling, cushioning, flushing,
lubrication and sealing (exam question).
There are two main methods of
lubrication, wet sump and dry sump.
The first is very simple, with the
engine oil in a sump under the
engine, in which the crankshaft and
other moving parts rotate, splashing
it all around (splash and mist).
Dry sump uses a tank outside the
engine, and oil is force fed around
under pressure where it is needed
(although wet sumps systems have
pumps, too). The scavenge pump
(which pulls oil from the engine) has
a greater capacity than the pressure
pump, to make sure the tank gets
filled properly. The filter will be
between the engine and the scavenge
pump. The oil cooler is between the
scavenge pump and the reservoir.
Pumps are usually mesh gear types:
Oils come in various thicknesses, or
viscosities, which measure resistance to
flow. The lower the viscosity
number, the thinner it is, so you
would use 120 oil in Summer, 100 in
Fall or Spring, 80 in Winter and 65
in the Arctic. To keep the oil thin, in
the cold, one trick is to pour a few
litres of petrol into the oil system
just after closing down at night, so it
is very thin in the morning and you
can start the engine. By the time the
oil has warmed up, the petrol has
evaporated and you can carry on
(but check your flight manual to see
if this Oil Dilution is acceptable for
your machine).
Oil is cooled by pumping it through
an oil cooler, which is just like a
58 Canadian Private Pilot Studies
radiator. An oil filter is used to trap
any impurities, and the pressure relief
valve is there to make sure it doesn't
get too high (if the pressure
increases, due to a blockage, maybe,
the valve opens and dumps the oil
back to the tank).
Chip Detectors are small magnets that
attract slivers of metal suspended in
the oil. Sometimes, they are
connected to a warning panel in the
cockpit in which a light glows if the
sliver makes a circuit across the
detector. It's always a good idea to
be prepared to land straight away if
you see a chip light come on, and
some flight manuals say do so
immediately. If the light is in a
transmission system, keep it loaded,
as unloading a disintegrating one has
been known to make it worse (in a
helicopter, make sure you land next
to a pub; the engineers like it better).
An engine that is not used enough
develops corrosion very quickly on
the inside, and rust flakes, which are
very abrasive, will circulate when the
engine is started, which is why you
have to change the oil even when
you don’t fly a lot. Another reason is
an increased water content, which
will have an acidic effect once it
mixes with the byproducts of
combustion, which is why you
should just pull the propeller
through several rotations if you
cannot fly. The most wear takes
place in the first seconds of a cold
start, after the oil has been allowed
to settle. Priming will wash whatever
oil is left off the cylinder walls, so
don't do too much, and maintain
minimum RPM to let the oil
circulate. The pressure will be high
just after starting, but will reduce to
　
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